Estrogen plays a pivotal role in the proper development and physiological function of both female and male reproductive organ. Environmental chemicals that interfere with the estrogen functions could cause various cellular abnormalities, thus endanger our health. In order to address the question on how environmental factors interfere with estrogen?s function, we need to understand the intricate molecular event associated with gene expression under the influence of estrogen. Estrogen regulates its target gene through estrogen receptors (ERs) by genomic and nongenomic pathways. With molecular approach, ER-mediated transactivation through the estrogen response element has been studied extensively. Many of the cofactors (coactivator and corepressors) that are implicated in estrogen signaling pathways were found and the mode of their interaction with ERs was elucidated. However, the molecular events associated with the ER-mediated transactivation of a natural promoter in the context of chromatin structure remain to be uncovered. Many estrogen response genes are differentially regulated by estrogen in spatial and temporal specific manner. Presence of the necessary factors in steroid hormone regulation are essential, the context of the gene in chromatin structure plays a major role in the estrogen responsiveness. We have been investigating the enhancer elements of the lactoferrin gene for many years. The gene encodes a nonheme iron-binding glycoprotein that modulates inflammatory and immune responses. The gene is responsive to estrogen, retinoic acid, EGF and FSK stimulation in tissue-selective manner. The lactoferrin gene promoters from both human and mouse contain similar estrogen response elements (EREs) but are surrounded by distinct DNA elements that bind different transcription factors. The arrangement of the DNA elements of the human and mouse lactoferrin gene together with the transactivating factors that interact either directly or indirectly with the DNA elements may have profound effects on the outcome of estrogen responsiveness in vivo. These unique arrangements of the enhancers/promoter of the mouse and human lactoferrin gene provide an opportunity to study the combinatorial regulation of two estrogen responsive natural promoters under the influence of estrogen, xenoestrogens and EGF in the cell. During the course of our present study, we cloned two transcription factors, an estrogen-related receptor alpha (ERRa) and Kruppel-like factor 5/intestine-enriched (KLF5/IKLF) that are participated in the regulation of lactoferrin gene promoter activity though estrogen and EGF signaling pathways, respectively. We showed that the ERRa modulates the estrogen responsiveness of the human lactoferrin gene. ERRs belong to a subfamily of orphan nuclear receptors closely related to the ERs. The ERRs share similar target genes and coactivators of ERs thus actively participated in the ERs-mediated estrogen action. Furthermore, ERRs were found to play important role in energy homeostasis. The close structural and functional relationship between ERs and ERRs suggest that these receptors are working cooperatively in estrogen action and may explain the pleiotropic effects of the estrogens. KLF5 is a key regulator in multiple morphogenetic programs. A severe heart defect was found in the KLF5 null mouse. While KLF5 may be crucial in development, we found protein-protein interaction between ERRa and KLF5 in the GST-pull down assay suggesting that the Kruppel family of transcription factors may modulate the estrogen responsiveness of the estrogen target genes.